1. Field of Invention
The present invention relates to optical interleaver. More particularly, the present invention relates to a wide passband interleaver.
2. Description of Related Art
The optical channel interleaver has been widely applied in a system or apparatus of Dense Wavelength-Division Multiplexing (DWDM), in which the DWDM technology is one of the best solutions to transmit a large amount of data. The interleaver and its application is shown in FIG. 1. In FIG. 1, the interleaver 100 receives a sequence of optical signals at the operation wavelengths, such as λ1, λ2, λ3, . . . λ79, λ80. The interleaver 100, for example, divides the 80 signals into an odd group of λ1, λ3, . . . λ79 and an even group λ2, λ4, . . . λ80. The signals of the odd group are output from one terminal while the signals of the even group are output from another terminal, in sequence. Basically, the signals of odd group are complementary to the signals of even group. In actual application, the signals of the odd group λ1, λ3, . . . λ79 are input to a DWDM 102, so as to convert the sequence signals into parallel signals in 40 channels. Likewise, the signals of the even group λ2, λ4, . . . λ80 are input to another DWDM 102, so as to convert the sequence signals into parallel signals in another 40 channels. Therefore, 80 channels of input signals can be converted into 80 parallel channels while the DWDM 102 may only operated in 40 channels.
The convention interleaver is usually formed by arrayed waveguide gratings (AWG), thin-film filter, or multi-lattice MZI (Mach-Zehnder Interferometer). For an interleaver by any manner, the concerned issue is how to design a good interleaver with an effective use on the bandwidth within each channel.
FIG. 2 is a drawing, schematically illustrating a conventional interleaver. In FIG. 2, the conventional interleaver includes MZI element and a ring resonant filter 126, coupled with the MZI element. The MZI element includes two direct optical couplers 120 and 128 and two optical routes 122 and 124 between the two direct optical couplers 120 and 128. The ring resonant filter 126 is coupled to the optical route 124. The sequence signals are input from the input terminal (Input). Due to effect of the ring resonant filter 126 with coupler 128, the signal responses at the terminals S1 and S2 are different in frequency. As a result, an interleaver can be achieved. However, the effective bandwidth ratio for the passband is just about 43% at 30 dB crosstalk operation in simulation.
In an ideal condition for operation, a perfect frequency spectrum property of the interleaver is that the signal is “1” for the passband and the signal is “0” for the stopband. After the optical signal passes this kind of filter for several times, precision of the signal can still remain. However, the usual filter is not in a perfect condition. Taking the usual one of Gauss-type filter as the example, after the optical signal passes this filter for several times, the frequency band becomes narrow, resulting in reducing on the bandwidth within the channel. When the frequency band of the optical signal in a single wavelength becomes narrow, the system needs to be designed with high precision, or the optical signal is very easily operated at the range of stopband of the filter, resulting in signal error. In order to meet the great amount of data transmission, each kind of data transmission system needs a specific design for the devices being used. This also causes the high fabrication cost.
For at least the above issues, it is strongly needed to design an interleaver with wide passband and even frequency spectrum property for practical use in optical data transmission system.